As known, in a packaging line of the above type packs are produced from a sheet packaging material, which is normally in the form of pre-cut blanks or a continuous strip and is subjected to a series of longitudinal folding and sealing operations to form a continuous tube of packaging material which, once filled, is sealed and cut transversely into individual packets.
The finished packets are then ordered to form two or more rows and are fed by a linear conveyor to a grouping and aligning unit, which is adapted to separate one or more, for example two, packet groups from the rest of the respective rows and to align the packet groups along a direction orthogonal to an advancement direction of the groups along the linear conveyor.
Downstream of the grouping and aligning unit, the packet groups first are moved through a wrapping station, where they are enveloped with a film of plastic heat-shrinkable material, and then are passed through an oven, where the film is heat-shrunk around each packet group so as to complete the formation of a respective multi-packet.
The wrapping station normally comprises a linear conveyor to advance the groups; a feeding device to feed sheet-cut film to an inlet of the linear conveyor; and a wrapping device having a plurality of bars which extend crosswise to the group advancement direction and are moved along an annular bar path extending around the linear conveyor. In particular, the bar path comprises a work stretch, along which the bars are moved in the same direction and at a higher speed than that of the groups so as to surmount and overtake them and resultingly wrap the film around the groups. The bar path further comprises a return stretch, along which the bars are moved below the linear conveyor in an opposite direction to the group advancement direction.
Though effective, the system described above has a drawback, due to the fact that at the inlet and outlet of the wrapping station and during the advancement through the wrapping station the packets are submitted to unavoidable stresses which may compromise the stability of the packets, in particular those with a relatively narrow rest base.